Even in the best of supermarkets, the fish available will quite often vary from day-to-day. However that should not always be a problem as most recipes will be just as good with an equivalent variety of fish.

Buying fish

Whole fresh fish have eyes that are bright and not sunken. The skin should be shiny and have a moist, firm appearance. Really fresh fish should have a pleasant sea fresh aroma. It should never have a strong ammonia smell or smell the least bit nasty.
Shellfish should always be tightly closed.

Rely on your senses, if doesn't look, feel or smell great, then you're probably best to leave it where it is.